1. Field of the Invention
Strong inorganic acid system for etching plates of magnesium and alloys thereof.
2. Description of the Prior Art
Magnesium (the term "magnesium" as used herein denotes magnesium with trace impurities and magnesium alloys, the same frequently being alloyed with zinc; such alloys include at least 70% magnesium and preferably at least 95% magnesium) printing and pattern plates are produced, by etching, from metal coated, in the present state of the art, with a light-sensitive coating which has been transformed by the action of light, with or without the assistance of heat, in certain areas, into an etch-resist pattern known as a photo-resist pattern, the remainder of the coating having been removed, as by dissolving, to leave bare metal areas.
An image is produced on a surface of a metal plate by a photomechanical process commonly practiced in the photoengraving art or by other standard means. The image consists of areas where the metal is protected by a photo-resist coating, and non-image areas where bare metallic surfaces are exposed. These bare metallic areas are etched to a sufficient depth to permit a printing process to take place or for the preparation of mats. Typical depths are in the range of from about 0.004" (e.g. for halftones) to about 0.040" for newspaper work and up to 0.150" for flexographic masters.
A plate having both the image (resist protected) and non-image (bare metal) areas is placed in a commercial etcher for etching. Various types of etchers are used, these all being of such character that droplets of an etching liquid are directed against the surface of the plate to be etched, preferably in such fashion that a large part of the droplets impinge on the plate in a direction having a substantial component perpendicular to said surface of the plate. Typical etching machines include those in which the droplets are formed and flung by paddles which successively dip into an etching bath and then are raised above it, turning about a horizontal axis; those in which the etching liquid is sprayed out of nozzles; and those in which streams of the etching liquid which break up into droplets are jetted from orifices in a revolving hollow shaft. Etchers of the foregoing type are shown, by way of example, in U.S. Pat. Nos. 2,669,048, 3,402,083 and 3,689,333.
Conventional magnesium etching liquids have as the primary etching agent a strong inorganic acid such as nitric acid, sulfuric acid or hydrochloric acid, nitric acid currently being the etching agent of choice. However, the action of such a strong acid on the exposed bare metal areas is omnidirectional, although the desired direction of etch is perpendicular to the surface to be etched and preferably is such that the sidewall (shoulder) beneath the non-etched (imaged) portion slopes (banks) preferably steeply, toward the etched area so as to strengthen the support for the imaged area; this is particularly desirable where the imaged area is small as, for example, in halftone, fine line and closed letter geometries. The omnidirectional etching action of strong inorganic acids results in lateral etching which undercuts and weakens the imaged areas and which even undercuts the protective resist so as to reduce and, on occasion, obliterate imaged areas.
To prevent this and to obtain a desired angle of bank, it has been the practice to include various additives in etching liquids, these including strong inorganic acid etching liquids for magnesium. In other words, it is the function of these additives to limit the direction of etching to the perpendicular or slightly off perpendicular (in favor of wider bases for the imaged areas) and thereby prevent lateral etching. Thus, sidewalls that form around the imaged areas are contiguous with the image, creating both the line and the halftone imaged areas which largely constitute a composite magnesium printing or pattern plate.
One of the conventional components of a commercial filming agent (a filming agent is a mixture of water-immiscible liquids, surfactants and coupling agents which is added to an aqueous etching agent to obtain a suitable angle of bank; "coupling agent" is a term which denotes a material having a solubilizing and/or dispersing effect and which specifically will render water-soluble and/or water-dispersible a bath additive that is not so normally characterized) is a water-immiscible hydrocarbon liquid, examples of which are gasoline, benzine, kerosene, turpentine, diethylbenzenes, coal oil and lubricating oils. Another conventional component is an anionic surface-active agent, e.g. a sulfonated surfactant, or a mixture of such agents. Still another component is a coupling agent or a mixture of coupling agents.
The filming agents which have been used conventionally, of late, in baths that etch magnesium in order to produce letterpress and pattern plates do not employ water-immiscible hydrocarbon liquids, except occasionally to a minor extent. Such conventional filming agents for magnesium eliminate the water-immiscible hydrocarbon liquids and, instead, employ fatty monocarboxylic acids that are in liquid form in the bath. These fatty monocarboxylic acids contain from 6 to 26 carbon atoms, are saturated or unsaturated, are unsubstituted or substituted with alkyl, aryl and/or alkylaryl groups, and have straight or branched chains. Such fatty monocarboxylic acids have been observed to control the etching depth of non-imaged areas alongside fine lines and halftones and to prevent undercutting of the imaged areas in these critically important portions of magnesium plates.
More recently, organic polycarboxylic acids such as dicarboxylic and tricarboxylic acids, examples of which are adipic, malic and citric acids (see U.S. Pat. Nos. 3,053,719 and 3,152,083) have been used as a component of filming agents for magnesium etching liquids. These were considered essential to obtain a better quality of etching in a nitric acid etching liquid for magnesium; but they were susceptible to oxidative degradation brought about by the nitric acid etchant. The degradation was not rapid but, in practice, as an etching liquid was used over a period of time, the degradation caused the performance of the etching liquid to deteriorate gradually. It was necessary for operators to detect such degradation before the results of etching became unsatisfactory and plates had to be discarded. When the degradation was observed the operator added to the nitric acid etching liquid other surfactants or components which counteracted the undesirable activity imparted by the degradation products of the polycarboxylic acids. To operate with such polycarboxylic acids entailed a certain amount of operator judgment, experience and skill.
The degradation even occurred when a nitric acid etching liquid containing polycarboxylic acids stood idle for several hours as, for example, over night, or for a few days such as over a weekend or holidays. Here, too, the degradation products of the polycarboxylic acids deteriorated the etching liquid and, as a matter of practice, frequently caused the liquid to be useless because of lowering of the protective action of the filming agent when the etching process was started up after a period of inactivity.
Another problem which has plagued the art in this field is that where water-immiscible hydrocarbon liquids were used in conjunction with sulfonated surfactants as the protective components of a filming agent, rather large amounts of the blends of these compounds had to be employed. This was rather expensive; moreover, the bulk of the filming agent led to high shipping and storage costs.
It also has been observed that the emulsified blends of water-immiscible hydrocarbon liquids and surfactants which produced a semi-stable emulsion that formed a protective film on the areas to be etched adjacent the lateral edges, fine lines, lines defining closed letter areas and delicate halftones tended to soften cured photo-resists that had been formed and which were supposed to protect the imaged areas against attack by the etching agent. Sometimes the softening was so extreme that the photo-resist stripped from the metal surface of the magnesium plate during etching and, obviously, where this occurred, the integrity of the image was destroyed.
There has been a further problem that applies with particular force to so-called "combination" plates which in the art are referred to as having "mixed geometries", these being plates with different types of configurations to be etched thereon such as halftones, line work, conventional printing, bold type and reverse printing which generally are characterized by the removal of considerably different amounts of metal per unit area.
It has been observed with current etching baths that the slopes of the sidewalls vary with the extent of metal removed in a unit area. Thus, where the amount of metal removed was small, the slopes of the sidewalls were steep [tight] and, conversely, when the amount of metal removed was large, the slopes of the sidewalls were less steep [broad]. The slope of the sidewall is generally known as a "shoulder angle", this being the angle between the sidewall and an imaginary line perpendicular to the original surface of the plate. Investigation has shown that in modern etching systems the sidewalls in zones where small amounts of metal are being removed are cooler than the sidewalls where large amounts of metal are being removed, due, it is believed, to the fact that where a small amount of metal is being removed the heat produced by etching is less than where a large amount of metal is being removed. Rough measurements in different areas indicate plate temperatures in the range of 100.degree.-200.degree. F. where a small amount of metal is being removed, and plate temperatures in the range of 200.degree.-400.degree. F. where a large amount of metal is being removed. Higher plate temperatures appear to have two different effects. One is a faster rate of etching. The second is an enhancement of the protective action of the protective film on the sidewalls.
As a practical matter, this has created difficulties. One has been that it is generally desirable to have a substantially uniform shoulder angle for the entire plate, and this has not been possible with combination plates because where the plate was hot the enhanced protection of the sidewall film broadened the shoulder angle. Conversely, in a cooler area with a weaker film the shoulder angle tightened or even undercut. Therefore, the practice has been to group plates according to the geometries of the overall photoresist patterns so that the shoulder angles could be held relatively constant and adjustments were made in bath temperatures, concentrations and paddle speeds to obtain a desired plate temperature and, hence, a desired angle for a given type of photoresist pattern.
On the other hand, where it has not been feasible to separate combination plates into groups, if a desired shoulder angle is obtained for one portion of the plate there may be undercutting at another portion of the plate, and if the shoulder angle is proper, say, for a cool area of the plate, the shoulder angle may be too broad for a hotter area of the plate.